The use of cryogenic probes in surgical or percutaneous transcatheter applications has been in existence for quite some time. There are several designs of cryorefrigerant systems for medical applications: Joule Thomson (with or without phase change) (“JT”) and circulating liquid (“CL”) where cooling occurs through direct heat transfer without a Joule Thomson effect. For cardiovascular applications, designers may be concerned with maximizing cooling performance while at the same time detecting and/or preventing fluid egress from the probe since this may result in catastrophic gas emboli in the bloodstream.
In a JT system, the fluid (gas or liquid) flows though an injection line to the cooling tip and undergoes a rapid pressure drop, and potentially a phase change, expansion at the nozzle tip of the injection line. It is this expansion, called Joule Thomson effect, with or without phase change that is endothermic and creates cold in the surrounding region. A number of systems have been designed to detect and prevent gas egress in such cases: double balloons, tip pressure containment and tip pressure detection.
In a CL system, a refrigerant (usually liquid) flows though the injection line and returns through a return line. The tip is cooled through a direct heat exchange between the injection line and the inner surface of the tip. Unlike the JT system, the refrigerant does not undergo a Joule Thomson expansion inside the tip and cooling occurs through direct heat transfer. Early CL systems used saline or other materials that are liquid at atmospheric room temperature. New CL systems now under development, as in U.S. Pat. No. 7,083,612 to Littrup, are using high pressure fluids such as Nitrogen (so called Critical Nitrogen) or other compressed liquid gasses in the injection line. These new systems have the potential to be much more powerful but also carry the added risk for cardiovascular applications due to high pressures and associated leaks leading to gas emboli entering into the bloodstream.
Accordingly, a need exists for an improved system design for example, to an efficient heat transfer at the tip and to both monitor and contain a leak in the system.